The present invention relates to an evacuating apparatus for use to exhaust a vacuum chamber in the semiconductor manufacturing plant.
In the semiconductor vacuum devices, it is particularly important that an evacuated chamber can attain a degree of vacuum of about 10xe2x88x923 Pa, and oil molecules must not enter the evacuated chamber. Thus, as a vacuum pump to meet such demands at a single stage, a screw vacuum pump (JP-B-7-9239) has been proposed which can exhaust the chamber from the atmospheric pressure to about 10xe2x88x923 Pa at a single stage (with a high compression ratio and a wide operable pressure range), and is oil free.
However, the screw vacuum pump had the following intrinsic problems.
(1) The screw vacuum pump is small in conductance because a thread groove is used to receive and transfer molecules of gas to be exhausted. Accordingly, the pumping speed is slow in a molecular flow range.
(2) The screw vacuum pump is necessary to have a clearance between mating faces of the male and female screws, and between the outer periphery of a screw and the inner periphery of a housing. Accordingly, the vacuum sealing ability is bad, which has an adverse effect on the ultimate vacuum.
(3) The screw vacuum pump has a bad vacuum sealing ability, as described above, and when used as a roughing vacuum pump, takes a large motive power (power loss) to recompress and discharge a back streaming of air from the atmosphere side. In particular, for the screw vacuum pump having a high pumping speed, the total amount of clearance as defined in (2) becomes large, resulting in a great tendency of motive power loss. Further, when a screw pump is used as a roughing vacuum pump, the screw pump produces a large power loss, which is caused by a difference in pressure between the suction side and the atmosphere side, even though a necessary degree of vacuum has been already reached on the suction side.
For the above-mentioned problems intrinsic to the screw vacuum pump, the following solving means has been conventionally proposed.
(A) First, solving means for a problem of conductance of the item (1) has been proposed in which the screw vacuum pump is used as the roughing vacuum pump that is less problematical with the conductance, and the booster pump is a Roots vacuum pump having large conductance.
In this two-stage pump, however, because the Roots vacuum pump has a small compression ratio, the pumping speed of the screw pump as the roughing vacuum pump can not be made too small. Owing to the fact that the pumping speed of the roughing vacuum pump can not be reduced, it follows that the capacity of the motor for driving this roughing vacuum pump can not be reduced, and each motive power loss of (3) can not be decreased. (A problem of (2) still remains.)
(B1) Solving means of a problem regarding the sealing ability of (2) has been proposed in which a plurality of chambers for transferring the fluid are provided between the suction port and the exhaust port by providing a plural number of turns of screw in the screw pump used at a single stage, to enhance the sealing ability (JP-B-7-9239). However, such solving means has an increased axial length of the screw, so that the devices become larger. Further, the plural number of turns of screw will not simply lead to solving the problem (3).
(B2) Similarly, solving means of the problem regarding the sealing ability of (2) has been proposed in which a screw vacuum pump is used as the booster pump which is less problematical with the sealing ability and a diaphragm pump or oil-sealed rotary vacuum pump having good sealing ability is used as the roughing vacuum pump (JP-A-62-243982). Since the oil-sealed rotary vacuum pump is usually provided with a check valve at a discharge port, it is possible to prevent back streaming of the air from the atmosphere side, so that each motive power loss as in (3) can be reduced.
In such two-stage pump, however, since the diaphragm pump or oil-sealed rotary vacuum pump having good sealing ability is necessary to be used as the roughing vacuum pump, in a case of the diaphragm pump, for example, reaction products (which are produced from a reactive gas flowed through the evacuated chamber) are likely to remain in the inside of the pump. If the reaction products remain, the exhaust performance may degrade remarkably, and it takes a lot of time and cost for overhaul. Also, in a case of the oil-sealed rotary vacuum pump, there is the danger that the evacuated chamber may be contaminated with oil molecules, and there is the problem that the oil may degrade in short time owing to a reactive gas, or must be exchanged frequently.
(C1) Solving means of a problem regarding the motive power loss in (3) has been proposed in which a micro-pump having a very small pumping speed is provided on the exhaust side of the roughing screw vacuum pump (JP-A-7-119666, JP-A-10-184576). The pumping speed of this micro-pump is large enough to suck and exhaust the reactive gas of a minute amount (no more than 50 to 150 cc/min) flowed through the vacuum chamber (the pumping speed is one several hundredths or less that of the roughing vacuum pump). In other words, the pumping speed is set to be very small. Accordingly, since the inverse torque owing to the difference in pressure which acts on the micro-pump becomes also very small, the motive power loss becomes very small.
However, this solution is that the roughing screw vacuum pump exhausts continuously from the atmospheric pressure to a high vacuum state, i.e., from a viscous flow area of the gas to a molecular flow area. Accordingly, in order to improve the sealing ability in the viscous flow area (roughing exhaust), it is required that the number of turns of screw is increased, and the clearance between the screw and the housing is reduced. And in order to satisfy the pumping speed in the molecular flow area, a large gas transfer volume must be provided. Accordingly, the screw vacuum pump becomes large in the radial and axial directions, resulting in the severe problem of clearance variations owing to thermal expansion. Consequently, high precision machining of the screw and its screw accommodating chamber (housing) is necessary, leading to higher costs. Since the screw vacuum pump of large volume exhausts the gas near the atmospheric pressure, a motor for driving the screw vacuum pump must also have a large capacity.
(C2) Similarly, solving means of the problem of motive power loss in (3) has been proposed in which the screw vacuum pump is used at a single stage by having not only a plural number of turns of screw but also a small volume of the transfer chamber on the exhaust side, as shown in FIGS. 11 and 12. This conventional example will be described below to facilitate the understanding of this invention.
A rotor accommodating chamber 210b formed inside a housing 210 rotatably accommodates a main screw rotor 220 constituted of male and female screw rotors 220m and 220f having a ratio of teeth of 4 to 5, and a sub-screw rotor 230 constituted of another male and female screw rotors 230m and 230f having a ratio of teeth of 4 to 5.
If a motor 243 is rotated, the male rotors 230m, 220m connected to this motor 243 are caused to rotate, while at the same time the female rotors 220f and 230f are caused to rotate via the timing gears 241 and 242. In this way, if the main and sub rotors 220 and 230 are driven to rotate, the gas within the evacuated chamber is sucked through a suction port 210a into the inside of the housing 210, transferred and compressed, and exhausted to the outside through an exhaust port 210c. 
By the way, the motive power required for a positive displacement vacuum pump 200 at the exhaust operation can be divided into a transfer motive power for transferring a sucked compressed fluid to the exhaust port 210c, a volume compression motive power owing to the volume of a transfer chamber of the positive displacement pump 200 being smaller from the suction port 210a to the exhaust port 210c, a motive power for transferring a compressed fluid that has flowed back through the clearance formed between the main screw rotor 220 or the sub-screw rotor 230 and the housing 210, from the high pressure side or exhaust side to the low pressure side or suction side, to the exhaust port 210c again, and a motive power (hereinafter referred to as a motive power owing to a differential pressure) against a force applied from the compressed fluid owing to a pressure difference between the suction side and the exhaust side.
The proportion of the motive power required for the positive displacement vacuum pump 220 at the exhaust operation may be different depending on the pressure of compressed fluid near the suction port 210a or near the exhaust port 210c. For example, when a vessel (hereinafter referred to as an evacuated vessel) of a fixed volume having an internal pressure equal to the atmospheric pressure is exhausted through the suction port 210a by the positive displacement vacuum pump 200, the pressure of compressed fluid neat the suction port 210a decreases with time, finally down to the ultimate pressure. However, when a small amount of gas may flow into the suction port 210a, the compressed fluid near the suction port 210a does not reach the ultimate pressure, but becomes a certain degree of vacuum. Accordingly, at the start of exhaust, the compressed fluid near the suction port 210a and that near the exhaust port 210c are both equal to the atmospheric pressure, and the required motive power is mainly a volume compression motive power. However, when the gas within the evacuated vessel has reached the ultimate pressure or become a certain degree of vacuum, there is a large difference in pressure between the compressed fluid near the exhaust port 210c and the compressed fluid near the suction port 210a, and the required motive power is mainly owing to a differential pressure.
Usually, since the vacuum pump is used to keep a vessel of fixed volume in vacuum in most cases, the motive power required when the vacuum pump is operating, i.e., the consumption motive power is mostly occupied by the motive power generated by the differential pressure. Accordingly, the energy saving of the vacuum pump can be effected by decreasing the motive power owing to differential pressure.
Herein, assuming that the torque of rotor is T, the rotating speed of rotor is N, and the constant is a, the consumption power W owing to differential pressure of each of the male and female rotors such as a screw vacuum pump can be given by the following expression (1).
W=axc3x97Txc3x97Nxe2x80x83xe2x80x83(1)
Also, assuming that a pressure area at high pressure side converted in a direction parallel to an axis of rotation of rotor is A1, the average pressure at high pressure side is P1, the distance from the center of A1 area to the center of rotation of rotor is L1, the pressure area at low pressure side converted in the direction parallel to the axis of rotation of rotor is A2, the average pressure at low pressure side is P2, the distance from the center of A2 area to the center of rotation of rotor is L2, the torque T can be given by the following expression (2), where the high pressure side means the exhaust side and the low pressure side means the suction side.
T=A1xc3x97P1xc3x97L1xe2x88x92A2xc3x97P2xc3x97L2xe2x80x83xe2x80x83(2)
In the above expression (2), A1, A2, L1 and L2 can be varied depending on the structure of a vacuum pump. According to the expressions (1) and (2), the motive power W owing to differential pressure can be reduced by determining the structure of the vacuum pump so that the torque T be smaller.
However, in practice, A2 and L2 are dimensions which are necessarily determined if the pumping speed of the vacuum pump is set. When the gas within the evacuated vessel has reached the ultimate pressure or become a certain degree of vacuum, i.e., the pressure on suction side is lower to some extent, a force owing to the pressure of compressed fluid on suction side can be ignored. Accordingly, the motive power W owing to differential pressure can be decreased by reducing A1 and L1, i.e., the volume of the transfer chamber 230A (hereinafter referred to as an exhaust side transfer chamber) formed by a tooth space of the sub-screw rotor 230 and the housing 210 and in communication to the exhaust port 210c (atmospheric pressure).
However, in the conventional vacuum pump like the above, the outer diameter of the sub-screw rotor 230 that forms the exhaust side transfer chamber 230A and the inner diameter of the housing 210 were formed to be equal to the outer diameter of the main screw rotor 220 and the inner diameter of the housing 210, respectively. Therefore, it was difficult to reduce the volume of the exhaust side transfer chamber 230A to an optimal dimension, if the volume of a transfer chamber 220A (hereinafter referred to as a suction side transfer chamber) formed by a tooth space of the main screw rotor 220 and the housing 210 and immediately after having been blocked off the suction port 210a is designed to be great, to increase the design pumping speed (the value of gas transfer volume per revolution of an input shaft multiplied by a rotating speed per unit time of the input shaft).
That is, in the case of the screw pump, the gas transfer chamber is formed by mating the male and female rotors. Accordingly, in the conventional vacuum pump, since the outer diameter of the male and female rotors 220m, 220f forming the suction side transfer chamber 220A is equal to the outer diameter of the male and female rotors 230m, 230f forming the exhaust side transfer chamber 230A, an intermediate transfer chamber 230B having a lead angle xcex82 may be reduced by making smaller the lead angle xcex82 of the sub-screw rotor 230, as shown in FIG. 11, in order to reduce the volume of the exhaust side transfer chamber 230A. However, there is the working limitation on making the lead angle xcex82 smaller. Consequently, the volume of the intermediate transfer chamber 230B could be reduced to only about ⅓ the volume of the suction side transfer chamber 220A. Owing to the fact that the volume of the intermediate chamber 230B can not be reduced, the volume of the exhaust side transfer chamber 230A can not be also reduced correspondingly. More specifically, the volume of the exhaust side transfer chamber 230A could be reduced to only about ⅕ the volume of the intermediate chamber 230B.
When a Roots or claw vacuum pump is concerned, the width of rotor in the axial direction must be decreased to reduce the volume of the exhaust side transfer chamber, but there is the limitation to decrease the width of rotor in the axial direction. If the volume of the suction side transfer chamber is designed to be great to increase the design pumping speed, it is difficult to reduce the volume of the exhaust side transfer chamber to the optimal dimension.
In this way, in the screw vacuum pump as shown in FIGS. 11 and 12, it was difficult to reduce the volume of the exhaust side transfer chamber to the optimal dimension. Therefore, the motive power owing to differential pressure could not be decreased, and the energy efficiency was low when the pressure on the suction side has reached the ultimate pressure or become a certain degree of vacuum.
Also, the axial length of screw is longer, leading to larger devices, as described in (B).
As described above, in the conventional evacuating apparatus using a screw vacuum pump, means for solving individually the problems intrinsic to the screw pump, i.e., concerning the conductance, sealing property, and consumption power, has been proposed, but there was no means for solving all the problems, and on one hand, such solving means gives rise to the new problem of larger devices or troublesome maintenance.
The present invention aims at solving the problems of such an evacuating apparatus using a screw vacuum pump.
In order to solve the above-mentioned problems, the present invention provides an evacuating apparatus having a roughing vacuum pump and a booster pump, each of which is constituted of a screw vacuum pump, wherein the design pumping speed (a value of a gas transfer volume per revolution of an input shaft multiplied by a rotating speed per unit time of the input shaft) of the roughing screw vacuum pump is sufficiently smaller than the design pumping speed of the booster screw vacuum pump, but adequate to be operable as the roughing vacuum pump, the number of turns of screw (the number of turns of screw having more teeth when the numbers of teeth for the male and female screws are different) for the roughing screw vacuum pump is greater than the number of turns of screw for the booster screw vacuum pump.
1) With the above constitution, since the screw vacuum pump having a high compression ratio as the general characteristic is used as the booster pump, a great pumping speed can be achieved as a whole system, even though the design pumping speed of the roughing vacuum pump is insignificant (small).
2) Further, the design pumping speed of the roughing screw pump is sufficiently smaller than the design pumping speed of the booster pump, but adequate to be operable as the roughing vacuum pump. Accordingly, the booster pump has no need of having the capability of exhausting from the atmospheric pressure on the suction side, and can have a compact and simple structure. On the other hand, the roughing vacuum can reduce the motive power loss owing to differential pressure in a state where the suction side has reached the ultimate pressure or become a certain degree of vacuum.
3) Since the design pumping speed of the roughing screw pump is small enough as described above, its screw radius can be reduced. Accordingly, the variations of clearance due to thermal expansion caused axially can be diminished to make the clearance developed radially smaller. Consequently, the total leakage space of gas is reduced, and the sealing property can be improved.
4) In this way, since the sealing property of the roughing screw pump can be made better, there is no need of increasing the number of turns of screw to ameliorate the sealing property and the axial length of the roughing vacuum pump can be lessened.
5) Since the sealing property of the roughing vacuum pump can be ameliorated, a high degree of vacuum can be attained, and the axial length of the booster pump can be reduced, even if the number of turns of screw for the booster pump is small or the clearance between the screw and the housing is poor in precision.
6) Since the number of turns of screw for the booster pump can be reduced, the axial length may not become excessive by raising the lead angle of screw for the booster pump to increase the conductance.
7) Since the screw vacuum pump of simple structure is adopted for both the roughing vacuum pump and the booster pump, the exhaust passage is simpler and shorter. Accordingly, reaction products are unlikely to clog in the exhaust passage, and even if they clog or stick together, they can be easily removed and the easy maintenance is effected.
In an evacuating apparatus of the present invention, the design pumping speed of the roughing screw vacuum pump is ⅕ to {fraction (1/100)} the design pumping speed of the booster screw vacuum pump.
With this constitution, the evacuating apparatus can be surely provided having a higher energy efficiency than the conventional one. The smaller the design pumping speed of the roughing screw vacuum pump with respect to the design pumping speed of the booster screw vacuum pump, the lesser the consumption power. But if the design pumping speed of the roughing vacuum pump is too low, there is the risk that the exhaust time is extended in a transient period where the evacuated vessel is exhausted from the atmospheric pressure to the ultimate pressure. Accordingly, in consideration of both the consumption power and the exhaust time, the design pumping speed of the roughing vacuum pump was made ⅕ to {fraction (1/100)} the design pumping speed of the booster pump.
In the evacuating apparatus of this invention, the number of turns of screw for the booster screw vacuum pump is substantially one, or such that at least one gas transfer chamber which is in communication with neither the suction port nor the exhaust port of the booster pump is formed.
With this constitution, the axial length of the booster screw vacuum pump which may greatly affect the dimensions of the device can be substantially minimum, and the device can be made smaller.
In the evacuating apparatus of this invention, the number of turns of screw for the roughing screw vacuum pump is 3 to 10.
With this constitution, the sealing property of the evacuating apparatus can be maintained excellent as a whole, even if the sealing property of the booster screw vacuum pump may not be ameliorated, and the axial length of the roughing vacuum pump does not becomes too excessive.
In the evacuating apparatus of this invention, the screw lead angle of the booster screw vacuum pump is larger than the screw lead angle of the roughing vacuum pump.
With this constitution, the axial length of the booster screw pump is greater correspondingly with the lead angle, but the conductance can be increased. On one hand, the axial length of the roughing screw pump does not become greater.
In the evacuating apparatus of this invention, the roughing screw vacuum pump is only driven until the suction side pressure of the booster screw vacuum pump falls from the atmospheric pressure to about 13,300 Pa, and the booster pump starts to be driven when the suction side pressure of the booster screw vacuum pump has fallen below about 13,300 Pa.
With this constitution, the motive power required to drive the booster pump may be small, and the driving motor may have a small capacity.
In the evacuating apparatus of this invention, a driving motor for each of the booster screw vacuum pump and the roughing screw vacuum pump is rotated at as high a rotating speed as possible as far as the motor is not overloaded, to shorten the exhaust time, in a range where the suction side pressure of the booster screw vacuum pump is relatively high. When the suction side pressure of the booster screw vacuum pump has reached the ultimate pressure or become a relatively low pressure, the rotating speed of the driving motor for the booster screw vacuum pump is reduced to the lowest rotating speed to maintain a degree of vacuum required for the evacuated chamber, and the rotating speed of the driving motor for the roughing screw vacuum pump is reduced to as low a rotating speed as possible in a range where the back pressure of the booster pump can be maintained below its critical backing pressure, so that the necessary motive power is reduced.
With this constitution, the pumping speed in exhausting the evacuated chamber from the atmospheric pressure can be increased, and the consumption power can be reduced.
The present disclosure relates to the subject matter contained in Japanese patent application Nos. Hei. 11-326276 (filed on Nov. 17, 1999), and 2000-213110 (filed on Jul. 13, 2000), which are expressly incorporated herein by reference in their entireties.